And harold w



19, 19290 P. lG ET nLEcTnIcAL INSTRUMENT Filed May 5, 1926 2 Sheet-Sheet l llllllmlllllllhh A TTORNEV Patented Mar, `149, 1929.

UNITED STATE PATENT OFFICE'.

PAUL G. EDWARDS, OF BROOKLYN, NEW YORK, AND HAROLD W. HERRINQTON, OF UPPER MONTCLAIR, NEW JERSEY, ASSIGNORS TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

ELECTRICAL INSTRUMENT.

Application iled'May 5, 1926. Serial No. 106,992.

This invention relates to electrical inst-rments, and particularly to instruments of the moving coil type capable of great sensitivity.

Instruments of the moving coil or 'dArson val type comprise a coil of wire suspended by means of a fine wire between opposite poles of a magnet. Generally, the moving coil has its axis perpendicular to the lines of force of the magnetic field. Current is led into the 1o coil by means of the suspension wire and leaves the coil by means of a flexible wire usually attached underneath the coil. In instruments of this type there are deflections of the moving coil proportional to the magnitude of the current flowing therethrough. In other words, when the current in the coil increases, the deflection increases, and vice' versa.

,In this invention there will be disclosed an electrical'instrument of the moving coil type in which, in addition to the magnetic field' established by the flow of current through a suspended moving coil, there is an electrostatic field also acting on the moving coil. In the particular type of instrument shown herein, for the purpose of illustration, the electromagnetic force co-operates with the electrostatic force and works in opposition to the force due to the suspension of the moving coil, but it is to be remembered, that these three forces, or more generally, the equivalents of these forces, may be combined in any manner whatsoever within the scope of this invention. j

It is an object of this invention to provide an electrical instrument which will give deflections proportional to the difference between the mechanical torque of a suspended movable coil and the electrostatic torque 'aeting on the moving coil in response to the charged conditions existing between the movf ing coil and the poles of an electromagnet, or

other suitable metallic shield.

It is another object of this invention to provide an electrical instrument for determining thefcharacteristics of a source of voltage by balancing the electrostatic forces established by the source of voltage'against the electromagnetic forces established by the flow of current from another source through a movable coil, the magnitude of the current flowing through the movable coil being varied until the movable coil is brought back to. an .arbitrary and predetermined deflection.

While the nature of thel invention'will be pointed out with particularity in the appended claims,the invention itself, both as toits further objects and features, will be better understood from the detailed description hereinafter following when read in connect1on with the accompanying drawing, in'

of iron. Within the loop 1 itself there is another core of iron 4 which decreases the magnetic gap between pole pieces 2-and 3. A shield 5, which may be of brass, copper or other conducting material, is fastened to the pole pieces 2 and 3 by means of a suitable cementing material which also insulates the shield 5 from the pole pieces 2 and 3, yet it .will be understood that any other shield may be mounted in any manner whatsoever be# tween the pole pieces 2 and 3 within the scope of this invention. The loop 1 is arranged so as to revolve between the pole pieces 2 and 3 and over and about the core of iron 4. A

source of electromotive force 6 is provided, having one terminal vconnectedto the shield 5 and the loop 1 and the other terminal connected to the'core of iron 4. A coil of wire 7isfixed on the core of iron having the pole pieces 2 and 3, and this coil of wire is energized by the llow of current from a -source 8 connected to its terminals. y

-Inasmuch as thereare like charges on the loop land the shield 5 there will be a force of repulsion which will tend to move the loo 1 away from vthe shield 5, and inasmuch as there are unlike charges on the loop 1 and the core of iron 4 there will be a force of attraction tending to move the loop 1 toward loo the core of iron 4. Since these forces of ati former position `a distance At this anglev cated by theA reference character Z1 there would be a'resultant torque due to these forces of attraction and repulsion which would tend to produce rotation of the loop, the torque being proportional to the sine of the angle of deliection A. However, it will be understood of deflection the distance between the ,loop

and its virtual image will be increased by AD. By virtue of the displacement of the loop-the force of repulsionbetween the loop and the shield 5 will be decreased. Atthis same angular displacement the distance between the loop and the surface of the .core of iron l adjacent thereto will be decreased by s and the distance of the loop from its virtual image will be decreased by AD.

Therefore, the force of'attraction between the loop of wire and the pole piece Ltwill be ini creased.

Furthermore, since only Ver small angles are lnvolved herein, the net e ect of revolving the loop through an angle A will be` equivalent to the application of a force having a definite magnitude acting with an arm proportional to the sine of the an le of deiection, producing rotation about t e axis of the loop. Since the sine of a very small angle isproportional to the angle itself, the electrostatic torque will also be proportional to the angle of deflection.

When a conductor 1n an electricalfield Y tends to exert a force upon another, either of attraction or of repulsion, as found, for eX- ample, in an electrical condenser, the force is given by the well-known expression l' v2 dc i f-'i' (1) In this expression, lv is the magnitude of the electromotive force between the conductors, c is the capacity between the conductors and do represents the change in capacity when one of these conductors moves a dista-nce lm toward or away from the other conducto-r.

In reference to the arrangement disclosed in' Fig. 1, the capacity is equivalent to that of a cylinder having,I a' diameter equal to the diameter of the loop 1, to a plane, the plane being the shield 5 adjacent tothe pole piece 2. Assumingay cylinder of infinite length and a plane of -infiniteextenm the capacity bei tween the cylinder and the plane is the same .as the capacity to neutral of the cyllnder to its virtual image. This capacity is given by the well-known expression 2 cosh -d- In this expression, la is thespecilic inductive capaclty (the speciic inductive capacity of a1r is l) d is the diameter of the cylinder, or

of loop 1, and D is the distance between the l centers 'of the cylinder and of .its virtual image.

In order to simplify the mathematics which j aid in understanding'the principles of the invention, let

Vy=c, :v-g and =cb Then ycosh-lav U or A 761 h- 4 a: cos y y() From the definitions of hyperbolic functions the following expression is obtained:

Then the derivative of m with respect to yis shown by the following expression:

By inverting expression (6), and from the ico definition of the hyperbolic function of .the

sine, the following expression is obtained:

Zi/ y2 Z dx k1 sinh l y Substituting for y its value obtained fromv the expression (3) given hereinabove, its follows that l en 'e d [(cosh 1:0) 2] [sinh (cosh "19;)1

Substituting for y xand la, their former pl values, then Expression (1) may now be written in the following form:

fw l 102k i n y 4 cosh 1%) (sinh cosh d?) (10) A The force given by expression (10,) acts together with a simklr force on the side of the loop ladjacent to t e pole piece 3 to forni` a couple having an arm proportional to the sine of the angle of delection. Since for small angles the ratio Tlis small, and since These conclusions are arrived at graphi-v cally in Fig. 2, which shows a.y number. of

curves in which the relative forces, as deter-l mined from expression (10) given hereinabove, are plotted as ordinates against angu- -lar deflection as abscissae. Curve 21 represents the increase in the force due to electrostatic attraction between the loop andv the core of iron 4 placedbetween the pole pieces of the electromagnet, as the angle of deflection increases. Curve 22 represents the de-f crease in the force of electrostatic repulsion between the loop of wir-e and the shield 5 touching the 4pole piece adjacent thereto as the angle of deiection increases. For the particular case to'which curves 21 and 22 apply, a curve 23 Acan be drawn proportional to the algebraic sum of the ordinates of curves 21 and 22. Curve 23 represents, therefore, the average of the electrostatic forces ot attraction and repulsion and indicates, by its slightly rising characteristic, that the electrostatic force of attraction is greater than the electrostatic force of repulsion. It is to be particularly noted that upon the defiection of the loop or movable coil the resultant increase of forces acting directly on the movable coil, as shown by curve 23, may become completelyneutralized by equal and opposite changes in electromagnetic forces produced by sending current through the movable coil. In other words, current maybe sent through the movable coil and a rotation will lie produced by the resultant electromagnetic torque, and this resultant' electromagnetic torque may be made to vary at the same rate that the resultant curve 23 varies. Consequently, the changes in the electrostatic forces may be made equal and opposite and may be neutralized by the changes produced in the electromagnetic forces as the movable coil is rotated or deliected. It will be apparent that curve 24 may represent the rate of change of the electromagnetic force with deflection, as .current is made to ow. through the movable coil.A Thus, curves 23 and 24 may neutralize each other so that there is no net change inthe resultant of the combined changes` in the electrostatic and the electromagnetic forces when considered' together. in view of the chan es produced in the deflec- ,tion of the mova le coil. The straight-line curve 25 represents the resultiint neutralized condition of the changing forces with rotation. Consequently, in an instrument having the characteristics alreadydescribed, the

inal deflection of the movable coil or loop of wire will be directly proportional to the current flowing therethrough.

It will be understood, however, that characteristics of the same general nature as those shown in Fig. 2 may be obtained for a movable coil known in the art as a flat coil. The force tending to produce rotation of a flat coil is given by the expression The capacity between the fiat coil and the shield touching the adjacent pole piece is given by the expression In expressions (11)- and (12), is the area of the face of the flat coil exposed to the adjacent pole piece and is the distance between this face of the fiat coil and the adjacent p ole piece. In these expressions the capacity between the coil and the adjacent pole piece varies with changes in the distance .c to that pole piece, and the capacity does not vary by'realson of its rotation. l

It will be understood, also, that the expressions (1) to (10), given hereinabove, apply to coils of any cross-sectional shape whatso- D ever if the ratio E is large. In such expressions the value of Z is obtained by dividing the perimeter of the cross-section by 1r.

Fig. 3 shows a number of curves which bear out the fact that the sensitivity may be varied by changing the electromotive force applied to the various elements. -Assuming that there is a linear relationship between the electrostatic torque tending to produce rotation and the angular deflection, then the line 3.1 will represent this electrostatic torque. Then line 32 -will represent the torque dueto torsion of the suspension of the loop. Obviously, the electrostatic torque will tend to neutralize the torque due to the torsion of the suspension of the loop. The dotted line 33 represents the electrostatic torque shown by line y31 plotted in the upper -part of the diagram, symmetrically arranged with respect to the horizontal axis. The line 34 will then represent the resultant of the electrostatic torque and the torque duelto the torsion of the suspension.

It' is conceivable that the electromotive i `coincide-with the horizontal axis. Equal electrostatie and suspension torques are particularly desirable in instruments such as ballistic galvanometers, etc.

If it is desired to use the arrangements of' this invention to measure the voltage characteristic of a source of current, the torsion of the suspension may be adjusted so as `to give zero torque at some predetermined angle. When the source of current is applied, a current will then be sent into the movable coil suilicient to hold the movable coil at the predetermined angle.` Current flowing through this movable coil will be proportional to the square of the voltage of the current applied. Consequently, this voltage can be determined from-.a calibration curve which shows the current in the movable coil plotted against the voltage applied. Inasmuch as the angle at which thecoil is initially placed may be varied, and the electrostatic force may be similarly7 varied by some suitable potentiometer arrangement, such an instrument will have a very lwide range.

Fig. 4 shows a perspective-of an electrical measuring instrument employing the principles of this invention. A laminated core of soft iron or steel 4l has a coil of wire 42 fixed thereon. The magnetic field due to this coil.

of wire affords a stronger flux density than is obtained with a permanent magnet and consequently, as is well known in the art, this instrument is much more sensitive than an instrument of a permanent magnet type. A movable coil 43 vhaving a terminal 44 is suspended in a metallic c se 45 from a spring support 46 by means of asuspension wire 47 which 'may be, for example, of phosphorbronze. Another spring support 48 is connected to the lower end of the movable coil 43 by means of another suspension wire 49 which may also be of phosphor-bronze. The metallic case 45 is permanently fiX'ed in position between the pole pieces 50 of the electromagnet. Another core of iron 51, which may also be made of soft iron or steel, is' fastened to an insulating support 52, the latter support holding all of the internal elements in position. A reiecting mirror 53 is carried by the suspension wire 47 so that the reflecting mirror may move when the movable coil 43 moves. A beam of light passes through a transparent glass window 54' and is reflected by the reiiecting mirror 53 through the transparent glass window 54 to a` graduated scale (not shown). The spring supports 46 and 48 may be considered the electrical terminals for the movable coil' 43.` Current enters the movable coil 43 by means of the suspension .wire 47 and leaves the movable coil by means of the suspension wire 49. Means may be provided for twisting the suspension wire 47 an amount sufficient to pla'ce the movable coil 43 at any predetermined angle with respect to the pole pieces 50. Y

The spring support 48 is connected to a terminal 55 of the metallic case 45..by means of a wire conductor. Another terminal 56 is provided at the core 51. A source of voltage 57 is connected between'terminals 55 and 56. It will be understood that squrce 57 is ordinarily a direct current voltage, although it may be a source of alternating current voltage when it is desired to measure alternating current characteristics.

While the invention has been disclosed in certainparticular embodiments for the pur` pose of illustration, it is not to be limitedby these embodiments, for it is capable of being set up in other and widely varied organizations without departing roin the spirit of.

the invention and the scope of the appended claims.

What is claimed is:

l. In an electrical instrument having a moving coil suspended between the pole `pieces of an electi'omagnet, the method of determining the magnitude of a. voltage, which consists in establishing an electrostatic field around the moving coil resulting ,in an elec-` trostatic torque tending to cause the rotation oif the moving coil, and balancing that electrostatic torque by an electromagnetic torque due to the fiow of current through the moving coil. Y

2. ln an electrical instrument, the combination of an electroinagnet, a moving coil suspended between the pole pieces'of the elec-- troinagnet, a source of electromotive force, said source of electroinotive force being connected between the moving' coil and the electifoinagnet so as to establishA an electrostatic torque tending to rotate the moving coil, and

means for transmitting current through the moving coil to establish an electromagnetic torque on the moving coil tending to bal--` ance the rotation due to the electrostatic torque on the moving coil.

3. In an'electrical instrument, the combination of an electromagnet, a moving coil suspended between the pole pieces of the electromagnet, the suspension of the moving coil producing a torque tending to rotate the moving coil, an electrostatic 'shield around the moving coil, a source of electromotive force, means for connecting the source of elec-tro- .inotive force between the electrostatic shield,

:Umwege` rection, electrically charging the moving coil with respect to the pole pieces of the electromagnet so as to produce a torque tending to revolve the moving coil inthe opposite direction, and transmitting current through the moving coil toproduce a torque tending to revolve the moving coil in the first mentioned direction.

5. Invan electrical instrument, the combination of an electromagnet, a coil of wire arranged to rotate between the pole pieces of the electromagnet, means for suspending the moving coil so as to produce a torque tending to revolve themoving coil in one direction, means for electrically charging rthe moving coil with respect to the pole pieces of the elec-- tromagnet so as to produce a torque tendingA to revolve the moving coil in the opposite direction, and means for transmitting current through the moving coil to produce a torque f tending to revolve the moving coil in the first mentioned direction.

6. In an electromagnet having a coil of wire arrangedso as to revolve between the pole pieces of said electromagnct, the method of measuring the current and voltage characteristics of a source of electricity, which consists in suspending the coil of wire so as to obtaima torque by reason of its suspension,

producing an electrical charge between the voil of wire so as to obtain an electromagletic torque tending to revolve the coil of wire in the opposite direction. 4

8. In an electrical instrument, the combination of two mutually reactive windings, a core of magnetic material having one of these windings fixed thereon, a .bar of magnetic material placed between the pole pieces of the core of iron, the other winding surrounding the bar of magnetic material located between the pole pieces of the' core of mag-v netic material and beiiig free to`m'ove between these pole pieces, -and a source of electromot1ve force havin one terminal connected to the movable co1 and the core of magnetic lmaterial and the other terminal connected to the bar of magnetic material.

9. Inan electrical instrument having a coil suspended so as to be 'movable about its axis,

the combination of means producing mechanical torque, means producing electromagnetic torque, and further. means producing electrostatic torque, said mechanical, electromagnetic and electrostatic torques simultaneously acting on the coil to produce its rotation aboutifits axis.

1,0. In an lelectrical instrument having a coil suspended so as to be movable about its a'Xis, the combination of means producing electromagnetic torque, and independent means producingelectrostatic torque, said electromagnetic and electrostatic torques acting simultaneously. on thecoil to produce its `rotation about its axis.

11. In an electrical instrument, the combination of a movable coil through which current may flow to 'cause its rotation, and independent means for establishing an electrostatic l'ield around said movable coil also causing its rotation.

12. An electrical instrument for measuringZ the characteristics of a source of current orvoltage, comprising a moving coil, means for suspending `the moving coil to eiiect a mechanical force tending to rotate the coil, meansfor transmittin current^through the moving coil for .establishing an electromagnetic eld also tending to cause its rotation, and other and independent means for establishing a substantial electrostatic lield around the moving coil to further cause .its rotation.

13. In -anelectrical instrument having a suspended movingcoil, the combination of a source of electromotive force, means whereby said source of electromotive force may set up an electrostatic vfield around the moving vcoil having a resultant force thereon acting to revolve it in onedirection, andwseparate and independent means for establishing an electromagnetic field around the moving coil acting to revolve it in the opposite direction.

In testimony whereof, we have signed our names to this specification this 4th day of Ma`y,1926. l

PAUL GnDwARDs." i HAROLD w. HERRINGTON. i 

